Mode-locked laser device

ABSTRACT

A mode-locked laser device includes a Fabry-Perot resonator, a mode-locking element disposed within the resonator, a solid-state laser medium disposed within the resonator, and exciting means for applying excitation light to the solid-state laser medium. The opposite ends of the resonator, the mode-locking element and the solid-state laser medium are disposed to provide an average beam diameter of lasing light of not more than 150 μm on the mode-locking element and an average beam diameter of the lasing light of not more than 200 μm within the solid-state laser medium.

This is a divisional of application Ser. No. 11/766,834 filed Jun. 22,2007. The entire disclosure of the prior application, application Ser.No. 11/766,834, is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mode-locked laser device, andparticularly to a mode-locked laser device provided with a solid-statelaser medium and a mode-locking element within a Fabry-Perot resonator.

2. Description of the Related Art

As a conventional technique for continuously generating optical shortpulse train, a mode locking method is used in which a solid-state lasermedium placed in a resonator is excited, for example, with asemiconductor laser, and phases of many lasing longitudinal modes arelocked. The mode locking method includes, for example, an active methodwhere an optical modulator is disposed in the resonator to apply lossmodulation, and a passive method where a saturable absorber whoseabsorption coefficient changes nonlinearly is disposed in the resonatorto achieve passive mode locking. A passive mode-locked laser deviceusing a saturable absorber has been proposed, for example, in“Diode-pumped mode-locked Yb₃ ⁺:Y₂O₃ ceramic laser”, OPTICS EXPRESS,Vol. 11, No. 22, pp. 2911-2916, Nov. 3, 2003, and International PatentPublication No. WO00/45480.

The mode-locked laser device typically uses a solid-state laser mediumdoped with a rare earth such as Yb (ytterbium) or Nd (neodymium). Forexample, a lasing threshold Pth of a laser device using a Yb-dopedsolid-state laser medium is expressed by formula (1) below, as describedin Appl. Opt., Vol. 36, No. 9, pp. 1867-1874 (1997):

$\begin{matrix}{{P_{th} = {\frac{\pi\;{{hv}_{p}( {\omega_{L}^{2} + \omega_{p}^{2}} )}}{4{{\sigma\tau\eta}_{a}( {f_{1} + f_{2}} )}}( {{Loss} + T + {2N_{0}f_{2}\sigma}} )}},} & {(1),}\end{matrix}$wherein ω_(L) represents the average beam radius (μm) of lasing light inthe solid-state laser medium, ω_(p) represents the average beam radius(μm) of excitation light in the solid-state laser medium, ν_(p)represents the frequency of excitation light, Loss represents theinternal loss of the resonator, T represents the transmittance of theoutput mirror, σ represents the stimulated emission cross section (m²),τ represents the fluorescence lifetime (ms), η_(a) represents theexcitation light absorption efficiency, N_(o) represents the amount ofdoped Yb ion, f₁ represents the upper laser level local distributionprobability, f₂ represents the lower laser level local distributionprobability, and h represents the Planck's constant.

As can be seen from formula (1), the lasing threshold can be lowered byreducing the beam radius (diameter) of lasing light and the beam radius(diameter) of excitation light within the solid-state laser medium. Toreduce the beam radius (diameter) of lasing light within the solid-statelaser medium, the resonator is typically designed so that a beam waistof the lasing light is formed within the solid-state laser medium.Further, in the passive mode-locked laser device using the saturableabsorber, it is necessary to form another beam waist of the lasing lighton the saturable absorber for achieving efficient mode locking. Since itis necessary to form two beam waists of the lasing light within theresonator, mode-locked laser devices disclosed, for example, in theabove-mentioned International Patent Publication No. 00/45480 and“Diode-pumped mode-locked Yb₃ ⁺:Y₂O₃ ceramic laser”, OPTICS EXPRESS,Vol. 11, No. 22, pp. 2911-2916, Nov. 3, 2003, employ three or moreconcave mirrors. This increases the number of parts forming the device,thereby making the device large and expensive, and poor in stability.

Further, in the lasing light obtained from the solid-state laser mediumdoped with Yb ion, three-level lasing is performed at the maximum peakin the fluorescence spectrum thereof, and therefore the lasingefficiency is significantly lowered by reabsorption loss due toelectrons distributed at the lower laser level absorbing the lasinglight. In order to avoid such reabsorption loss, it is necessary to fillthe upper laser level with electrons by performing high-densityexcitation to minimize the reabsorption of the lasing light. In a casewhere a semiconductor laser is used as an excitation light source, dueto a limitation in the power of commercially-available semiconductorlasers, it is necessary to increase laser density by reducing the beamdiameter of excitation light within the solid-state laser medium and toincrease the lasing efficiency by increasing overlap between theexcitation light and the lasing light within the solid-state lasermedium, in order to achieve high-density excitation. The reason forincreasing the overlap between the excitation light and the lasing lightwithin the solid-state laser medium is that, if the beam diameter of theexcitation light is smaller than the beam diameter of the lasing lightwithin the solid-state laser medium, a large reabsorption loss occurs atareas where no excitation light is present and the lasing efficiencydecreases. In contrast, if the beam diameter of the excitation light islarger than the beam diameter of the lasing light within the solid-statelaser medium, even areas where no lasing light is present, i.e., areaswhich do not contribute to lasing are excited and the lasing efficiencyalso decreases, and therefore sufficient high-density excitation may notbe achieved.

On the other hand, the width of light with a wavelength of 940 to 980 nmemitted by a current commercially-available high-power semiconductorlaser, which can excite the solid-state laser medium doped with Yb ion,is around 100 μm at the smallest. Therefore, in a Fabry-Perot resonatorformed by a small number of parts, it is necessary to excite thesolid-state laser medium via a resonator mirror. This increases thedistance from the condensing lens to the solid-state laser medium, andnecessitates a complicated excitation optical system to efficientlycondense the light to have a small diameter.

In a case of a four-level system laser medium such as a solid-statelaser medium doped with Nd ion, similarly to the above-described case ofthe solid-state laser medium doped with Yb ion, the lasing threshold canbe lowered by reducing the beam diameters of the lasing light and theexcitation light within the solid-state laser medium and increasing theoverlap between the lasing light and the excitation light. Therefore,such device also has a complicated resonator structure and a complicatedexcitation optical system.

As described above, conventional mode-locked laser devices have multipleconcave mirrors and thus have a complicated structure. They have pooroutput stability, and are large and expensive due to the large number ofparts. In addition, they use a complicated excitation optical system toimprove excitation efficiency, and this further increases the size ofsuch devices.

SUMMARY OF THE INVENTION

In view of the above-described circumstances, the present invention isdirected to provide a mode-locked laser device that is compact, has asimple structure and high output stability.

A first aspect of the mode-locked laser device of the present inventionincludes: a Fabry-Perot resonator; a mode-locking element disposedwithin the resonator; a solid-state laser medium disposed within theresonator; and exciting means for applying excitation light to thesolid-state laser medium, wherein opposite ends of the resonator, themode-locking element and the solid-state laser medium are disposed toprovide an average beam diameter of lasing light of not more than 150 μmon the mode-locking element and an average beam diameter of the lasinglight of not more than 200 μm within the solid-state laser medium.

It may be more preferable that the average beam diameter of the lasinglight on the mode-locking element is not more than 100 μm.

The “beam diameter” is defined by an area where the light intensity isnot less than 1/e² of the maximum intensity in the intensitydistribution at a cross-section that is perpendicular to a travelingdirection of the light. The “beam diameter” is not uniform but varieswithin the mode-locking element and within the solid-state laser medium(for example, different beam diameters are formed at the entering edge,the central portion, and the emitting edge), and the “average beamdiameter” is an average of beam diameters within the mode-lockingelement or the solid-state laser medium. It should be noted that in acase where the mode-locking element forms one of the ends of theresonator, the beam diameter of not more than 150 μm on the mirrorsurface of the mode-locking element is sufficient.

In the mode-locked laser device of the invention, the mode-lockingelement and the solid-state laser medium may be disposed in the vicinityof a beam waist of the lasing light.

A second aspect of the mode-locked laser device of the inventionincludes: a Fabry-Perot resonator; a mode-locking element disposedwithin the resonator; a solid-state laser medium disposed within theresonator; and exciting means for applying excitation light to thesolid-state laser medium, wherein the solid-state laser medium and themode-locking element are disposed in the vicinity of a beam waist oflasing light with a distance between the solid-state laser medium andthe mode-locking element being less than ½ of the length of theresonator.

In the mode-locked laser device of the invention, output light may havea lasing wavelength corresponding to a wavelength of a peak influorescence spectrum of the solid-state laser medium, at whichfour-level system lasing is performed. It should be noted that the“four-level lasing” includes “quasi-four-level system” that behavessimilarly. For example, a wavelength at which four-level lasing isperformed, such as lasing at a wavelength of 1050 nm by the Yb:YAG, isincluded. In the four-level system lasing, rate of decrease in lasingefficiency, which decreases as the beam diameter of the excitation lightwithin the laser medium increases, is smaller than that of thethree-level system, and therefore high lasing efficiency can be achievedwithout reducing the beam diameter of excitation light within the lasermedium as small as that required in the three-level system. Therefore,the four-level system lasing is more suitable for the mode-locked laserdevice of the invention.

The mode-locking element may be formed by a saturable absorber mirrorand may form one of the ends of the resonator.

The exciting means may apply the excitation light to the resonator froma direction crossing the optical axis of the resonator. In this case, adichroic mirror for reflecting the excitation light and transmitting thelasing light may be provided in the resonator, so that the excitingmeans directs the excitation light to the dichroic mirror and thedichroic mirror reflects the excitation light toward the solid-statelaser medium to apply the excitation light to the solid-state lasermedium, or the exciting means may direct the excitation light directlyto the solid-state laser medium.

Moreover, in the invention, it may be desirable that the resonatormirror, which forms one of the ends of the resonator, is provided with acoating that has a negative group velocity distribution to provide azero or less group velocity distribution for the entire resonator.

Further, the opposite ends of the resonator may be formed by a concavemirror and the mode-locking element, and the exciting means may applythe excitation light to the resonator from the back side of the concavemirror along a direction substantially parallel to the optical axis ofthe resonator. In this case, the length of the Fabry-Perot resonator maybe in a range from 1 mm to 14 mm.

The exciting means may be formed only by a semiconductor laser foroutputting the excitation light and a single lens for condensing theexcitation light onto the solid-state laser medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing the schematic structure of a mode-lockedlaser device of a first embodiment of the present invention;

FIG. 2 is a plan view showing the schematic structure of a mode-lockedlaser device of a second embodiment of the invention;

FIG. 3 is a plan view showing the schematic structure of a mode-lockedlaser device of a third embodiment of the invention;

FIG. 4 is a graph showing dependence of a mode locking threshold on abeam diameter of lasing light on a SESAM; and

FIG. 5 is a plan view showing a modification of the mode-locked laserdevice of the first embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail.

FIG. 1 is a plan view showing the schematic structure of a mode-lockedlaser device 1 according to a first embodiment of the present invention.The mode-locked laser device 1 includes a Fabry-Perot resonator 5, asolid-state laser medium 13 disposed in the resonator 5, a mode-lockingelement 15, and an exciting means 6 for applying excitation light 10 tothe solid-state laser medium 13. The mode-locking element 15 is asaturable absorber mirror, which forms one of the ends of the resonator5. The other end of the resonator 5 is formed by an output concavemirror 14. The resonator 5 includes no other mirror for reflecting thelasing light than the saturable absorber mirror 15 and the output mirror14 forming the ends of the resonator, and the resonator 5 is structuredas a linear resonator.

The exciting means 6 is formed by a semiconductor laser 11 foroutputting a laser beam which serves as the excitation light 10, and acondensing lens 12 for condensing the excitation light 10 onto thesolid-state laser medium 13. The semiconductor laser 11 is enclosed in acan package. The exciting means 6 directs the excitation light 10 to theresonator 5 from a direction crossing the optical axis A of theresonator 5. In this embodiment, the exciting means 6 directs theexcitation light 10 to the resonator 5 from a direction substantiallyperpendicular to the optical axis A. A dichroic mirror 16 that reflectsthe excitation light 10 into the resonator 5 and transmits lasing light7 is disposed with an inclination of 45° with respect to the opticalaxis A of the resonator. As the exciting means 6 directs the excitationlight 10 to the dichroic mirror 16, the excitation light 10 is reflectedby the dichroic mirror 16 and enters the solid-state laser medium 13.

The resonator 5 is structured such that only one beam waist of thelasing light 7 is formed within the resonator 5 (on the mode-lockingelement in this embodiment). The solid-state laser medium 13 and thesaturable absorber mirror 15 are disposed in the vicinity of the beamwaist such that a distance d between the solid-state laser medium 13 andthe saturable absorber mirror 15 is smaller than ½ of a resonator lengthL.

The resonator length L and a curvature of a concave surface 14 a of theoutput concave mirror 14 are set in a range where the beam waist of thelasing light is formed on the mirror surface 15 a of the saturableabsorber mirror 15, and a beam diameter D₁ of the lasing light on themirror surface 15 a is not more than 150 μm. The solid-state lasermedium 13 is disposed such that an average beam diameter D₂ (=2ω_(L)) ofthe lasing light within the solid-state laser medium 13 is not more than200 μm. More specifically, the beam diameter D₁ of the lasing light onthe mirror surface 15 a is in a range from 10 to 150 μm. The lower limitvalue of 10 μm corresponds to a limit in design, and the upper limitvalue of 150 μm corresponds to a limitation for obtaining necessarylight density for achieving efficient mode locking by the saturableabsorber mirror. The average beam diameter D₂ of the lasing light withinthe solid-state laser medium 13 is in a range from 50 to 200 μm. Thelower limit value of 50 μm corresponds to a limit in design of thestructure in which the resonator is formed by the output concave mirrorand the saturable absorber mirror and the beam waist of the lasing lightis formed on the mirror surface of the saturable absorber mirror, as inthis embodiment. Further, if the average beam diameter D₂ of the lasinglight exceeds 200 μm, the lasing threshold exceeds 100 mW and thisresults in lower efficiency.

The upper limit value of 150 μm for the beam diameter D₁ of the lasinglight on the mirror surface 15 a of the saturable absorber mirror 15 hasbeen found from the following consideration. In the followingconsideration, a semiconductor saturable absorber mirror (SESAM) is usedas the saturable absorber mirror 15.

In order to induce mode locking, the pulse energy within the resonatorneeds to exceed a mode locking threshold E_(p,c) expressed by formula(2) below (see J. Opt. Soc. Am. B, Vol. 16, 46-56 (1999)):E _(p,c)=(F _(sat,L)×πω_(L) ² ×F _(sat,A)πω_(sat) ² ×ΔR)^(1/2)  (2).

The above formula (2) includes five main parameters including thesaturation fluence of the solid-state laser medium (F_(sat,L)), thesaturation fluence of the SESAM (F_(sat,A)), the modulation depth of theSESAM (ΔR), the beam diameter of the lasing light within the solid-statelaser medium (ω_(L)), and the beam diameter of the lasing light on theSESAM (ω_(sat)). The mode locking threshold E_(p,c) can be reduced byreducing these parameters. Therefore, stable mode locking can beachieved with relatively small pulse energy within the resonator byreducing these parameters. Among currently known solid-state lasermedia, one having the smallest saturation fluence (F_(sat,L)) is Nd:YVO₄((F_(sat,L))=0.04 J/cm²), which is often used as the mode-lockingsolid-state laser medium since it provides a small mode lockingthreshold. Further, among current commercially-available SESAMs, onebeing usable with Nd:YVO₄ (1064 nm) and having the smallest saturationfluence (F_(sat,A)) and the smallest modulation depth (ΔR) is a SESAMmanufactured by BATOP, having F_(sat,A) of 120 μJ/cm² and ΔR of 0.3%.The combination of this SESAM and Nd:YVO₄ can provide the currentlysmallest mode locking threshold.

The graph of FIG. 4 shows dependence of the mode locking threshold andthe pulse energy within the resonator on the diameter of the lasing beamon the SESAM. Here, Nd:YVO₄ is used as the solid-state laser medium andthe above-described SESAM manufactured by BATOP is used as the SESAM,and the solid-state laser medium is disposed in the vicinity of theSESAM so that the beam radius of the lasing light within the solid-statelaser medium and the beam radius of the lasing light on the SESAM areabout the same (ω_(L)=ω_(sat)), i.e., the relationship between the beamdiameters D₁ and D₂ in FIG. 1 is D₁=D₂, to find dependence of the modelocking threshold on the beam diameter of the lasing light on the SESAMbased on the above formula (2). Further, the pulse energy within theresonator is found based on formula (3) below:

$\begin{matrix}{{E_{p,{in}} = {\frac{P_{out}}{T} \times \frac{L}{c}}}{{P_{out} = {\eta_{slope} \times ( {P_{i\; n} - P_{th}} )}},{\eta_{slope} = {\frac{T}{T + {Loss}} \times \frac{\lambda_{p}}{\lambda_{L}} \times \eta_{a} \times \frac{\omega_{L}^{2}( {{2\omega_{p}^{2}} + \omega_{L}^{2}} )}{( {\omega_{p}^{2} + \omega_{L}^{2}} )^{2}}}},{and}}{{P_{th} = {\frac{{hv}_{p}{\pi( {\omega_{p}^{2} + \omega_{L}^{2}} )}}{4\eta_{a}{\sigma\tau}} \times ( {L + T} )}},}} & {(3),}\end{matrix}$wherein P_(out) represents the average output, T represents thetransmittance of the output mirror, L represents the resonator length, crepresents the light velocity, η_(slope) represents the lasing slope,P_(in) represents the excitation power, P_(th) represents the lasingthreshold, Loss represents the internal loss, λ_(p) represents theexcitation wavelength, η_(a) represents the excitation light absorptionrate, ω_(p) represents the average beam radius of the excitation lightwithin the solid-state laser medium, h represents the Planck's constant,σ represents the stimulated emission cross section, and τ represents thefluorescence lifetime. Values for the respective parameters are: T=0.2%,L=0.03 m, Loss=0.6%, λ_(p)=808 nm, λ₁=1064 nm, η_(a)=0.99, ω_(p)=25 μm,σ=25.4×10⁻¹⁹ J/cm², and τ=90 μs. Further, in this calculation, as theexcitation light source, a semiconductor laser having maximum power of2.5 W, lasing wavelength of 808 nm and emitter of 50 μm manufactured byQuintessence Photonics Corporation, which has the highest luminanceamong current commercially-available semiconductor lasers, is used.

It can be seen from the graph that, when the lasing beam diameter on theSESAM is not more than 150 μm, the pulse energy within the resonatorexceeds the mode locking threshold and thus mode locking can beachieved. In other words, in order to achieve mode locking in theabove-described resonator structure, it is necessary to have the beamdiameter of the lasing light on the SESAM be not more than 150 μm.However, upon modularization (productization) for practical use, it isdesirable to design the beam diameter of the lasing light to be not morethan 100 μm to provide latitude of about ⅓ with taking tolerance intoaccount.

It should be noted that the average beam diameter D₂ of the lasing lightwithin the solid-state laser medium 13 is shown in FIG. 1 as the beamdiameter at the center of the solid-state laser medium for convenience,however, the average beam diameter refers to an average of beamdiameters of the lasing light that passes through the solid-state lasermedium within the solid-state laser medium.

Further, in this embodiment, each of the components 11 to 16 is disposedin a fixed position on the peltiert element 17. However, each of thecomponents 11 to 16 may be fixed on the peltiert element 17 via a metal(copper, for example) holder. A thermistor 18 for sensing temperature isfixed on the peltiert element 17. Based on the output from thethermistor 18, a temperature controlling circuit (not shown) drives thepeltiert element 17 to maintain the semiconductor laser 11, thecondensing lens 12, the solid-state laser medium 13, the output concavemirror 14, the mode-locking element 15 and the dichroic mirror 16 at apredetermined temperature.

The mode-locked laser device 1 of this embodiment can have a simplestructure having the mirrors only at the opposite ends of the resonator.Further, the exciting means can be formed only by a semiconductor laserand a single lens. Therefore, a compact and stable mode-locked laserdevice can be accomplished.

An example of more specific structure of the mode-locked laser device 1will now be described. As the solid-state laser medium 13, a Yb:Y₂O₃medium is used, which is produced by doping Yb ion to a ceramic mediumY₂O₃ (yttria), which is a substrate. The solid-state laser medium dopedwith Yb ion acts as a three-level system at a wavelength of the maximumpeak of the fluorescence spectrum thereof. However, in the mode-lockedlaser device 1, light with a wavelength (1075 nm) of a peak at a longerwavelength than the maximum peak wavelength, at which the solid-statelaser medium acts as a four-level system, is used as output light 8.

The Yb:Y₂O₃ medium 13 is doped with 10 at % of Yb ion and has athickness of 0.65 mm. The both sides of the Yb:Y₂O₃ medium 13 areprovided with a coating that well transmits either of the excitationlight 10 with a wavelength of 980 nm and the lasing light 7 with awavelength band of 1075 nm.

As the saturable absorber mirror 15, a semiconductor saturable absorbermirror (hereinafter referred to as SESAM) manufactured by BATOP, havinga modulation depth of 0.4% and a saturation fluence of 120 μJ/cm², isused.

The mirror surface (concave surface) 14 a of the output concave mirror14 has a curvature radius of 30 mm, and is provided with a coating thattransmits about 1% of the lasing light 7.

The output concave mirror 14 and the SESAM 15 are disposed such that theresonator length L defined between the mirror surface 14 a of the outputconcave mirror 14 and the mirror surface 15 a of the SESAM 15 is 30 mm(in the air). The Yb:Y₂O₃ medium 13 is disposed at a distance d of 6 mmfrom the SESAM 15.

As the semiconductor laser 11, a broad-area semiconductor laser having awavelength of 980 nm, an emission light width of 100 μm and a power of 2W is used. As the condensing lens 12, a lens that provides the beamdiameter of the excitation light of around 100 μm within the Yb:Y₂O₃medium 13 is used. The condensing lens 12 is disposed in the vicinity ofthe dichroic mirror 16 so as not to interfere with the optical path ofthe resonator. The condensing lens 12 condenses the excitation light 10so that the beam waist of the excitation light is positioned in thevicinity of the center of the Yb:Y₂O₃ medium 13 in the thicknessdirection of the medium.

The dichroic mirror 16 is a 1-mm square quartz plate having a thicknessof 0.3 mm and provided with a coating that well reflects the excitationlight 10 with the wavelength of 980 nm entering at an incident angle of45° and well transmits the lasing light 7 with the wavelength band of1075 nm entering at an incident angle of 45°. The dichroic mirror 16 isdisposed in the vicinity of the Yb:Y₂O₃ medium 13.

In this structure, the lasing beam diameter D₁ on the SESAM 15 is 64 μm,and the average lasing beam diameter D₂ within the Yb:Y₂O₃ medium 13 is146 μm.

In the mode-locked laser device 1, the light with the wavelength band of1075 nm, which is excited by the excitation light 10 and emitted by theYb:Y₂O₃ medium 13, resonates between the output concave mirror 14 andthe SESAM 15 and is mode-locked by the SESAM 15, and then is outputtedfrom the output concave mirror 14 as the output light (pulse laser) 8.The mode-locked laser device having the above-described structureprovides a recurrence frequency of 5 GHz, a pulse width of 1.5 psec. andan average power of 350 mW, where the average power=pulseenergy×recurrence frequency. The average power obtained in thisembodiment is 1.5 times efficient than the average power of aconventional device which has the structure similar to the device of thefirst embodiment and has the laser medium positioned at a distance of 15mm from the SESAM, which position is near the center of the resonator.In the conventional device, the diameter of the lasing beam within thelaser medium is as large as 300 μm or more. Although the samesemiconductor laser is used for outputting the excitation light, theconventional device uses complicated excitation optics as the opticalsystem for condensing the excitation light onto the resonator to providea condensed beam diameter of 100 μm within the laser medium. An averageoutput of the conventional device excited with the same excitation poweras that in the above-described embodiment is 220 mW.

In a mode-locked laser device 1′ shown in FIG. 5, in stead of the outputconcave mirror 14, an output concave mirror 14′ is provided which has acoating 19 that has a negative group velocity distribution to compensatea positive group velocity distribution within the resonator and providea state where the group velocity distribution for the entire resonatoris completely compensated (the group velocity distribution=0) or thegroup velocity distribution within the resonator is negative (the groupvelocity distribution<0). This allows induction of soliton mode locking,thereby achieving pulsed light having a pulse width of sub-picosecondorder.

For example, in a case where a positive group velocity distributionoccurs in the resonator of the device 1′ of FIG. 5, the output concavemirror 14′ having the coating 19 that has a group velocity distributionof −3000 fs² is used, and the group velocity distribution for the entireresonator of −2700 fs² and pulsed laser light having a pulse width of800 fs can be achieved. It should be noted that, for applying thecoating having a negative group velocity distribution, aconventionally-used multilayer film coating technique which is describedas a distribution compensating method in Japanese Unexamined PatentPublication No. 11 (1999)-168252, for example, can be used, and a filmstructure which is similar to a negative distribution mirror describedin R. Szipoecs, et. al., Optics Letters, Vol. 19, 201 (1994) can beused.

In general, a Yb-doped solid-state laser medium has a large F_(sat,L),and therefore mode locking is not achieved in a state where the modelocking threshold is high, the resonator length L is as short as 30 mm,and the pulse energy within the resonator is small. However, bycompensating the group velocity distribution within the resonator toform a state called soliton mode locking, the mode locking threshold ofthe Yb-doped solid-state laser medium can be made as low as that ofNd:YVO₄, and therefore mode locking can be achieved with the resonatorlength as short as 30 mm.

It should be noted that the dichroic mirror 16 may be disposed at anangle other than 45° with respect to the optical axis A, and/or may bedisposed between the Yb:Y₂O₃ medium 13 and the SESAM 15. Further, anelement for controlling polarization such as a Brewster plate may beprovided within the resonator, or the dichroic mirror may be providedwith a certain coating and disposed at the Brewster angle to be used asboth the dichroic mirror and the Brewster plate. Alternatively, theYb:Y₂O₃ medium 13 may be disposed with an inclination corresponding tothe Brewster angle with respect to the optical axis A.

Although the lasing light is outputted from the output concave mirror 14in the above-described embodiment, the lasing light can be outputtedfrom the semiconductor saturable absorber mirror by providing theconcave mirror with a coating that well reflects the lasing light andusing a transmission-type semiconductor saturable absorber mirror.Besides the SESAM, a saturable absorber mirror using a Kerr mode-lockingelement or a carbon nanotube can be used as the mode-locking element.

Next, a mode-locked laser device 2 according to a second embodiment ofthe present invention will be described with reference to FIG. 2. Thebasic structure of this embodiment is the same as that of the firstembodiment, and therefore the same components are designated by the samereference numerals and are not explained in detail. Explanation will begiven mainly on different points.

The mode-locked laser device 2 of this embodiment differs from themode-locked laser device 1 of the first embodiment in that the positionof the exciting means 6 is different and the dichroic mirror 16 is notprovided.

The exciting means 6 applies the excitation light 10 to the resonatorfrom a direction crossing the resonator axis A, and the excitation light10 obliquely enters the solid-state laser medium 13 directly withoutinvolving a dichroic mirror. In this case, it is necessary to adjust theincident angle and the condensing position of the excitation light toachieve optimal mode matching within the solid-state laser medium 13.

In the specific structure of the mode-locked laser device 2, componentsother than the exciting means 6 and their positions can be the same asthose in the mode-locked laser device 1 described above. Similarly tothe exciting means 6 in the first embodiment, the exciting means 6 inthe second embodiment can use the broad-area semiconductor laser 11having a wavelength of 980 nm, an emission light width of 100 μm and apower of 2 W, and the condensing lens 12 that provides the beam diameterof excitation light of 100 μm within the Yb:Y₂O₃ medium 13.

With this structure, a recurrence frequency of 5 GHz, a pulse width of1.5 psec. and an average power of 300 mW are obtained.

Next, a mode-locked laser device 3 according to a third embodiment ofthe present invention will be described with reference to FIG. 3. FIG. 3is a plan view showing the schematic structure of the mode-locked laserdevice 3 of the third embodiment of the present invention. Themode-locked laser device 3 includes a Fabry-Perot resonator 30, asolid-state laser medium 23 disposed within the resonator 30, amode-locking element 25, and an exciting means 20 for applying theexcitation light 10 to a solid-state laser medium 23. The mode-lockingelement 25 is a transmission-type saturable absorber mirror, which formsone of the ends of the resonator 30. The other end of the resonator 30is formed by a concave mirror 24. The resonator 30 includes no othermirror for reflecting the lasing light than the saturable absorbermirror 25 and the output mirror 24 forming the ends of the resonator,and the resonator 30 is structured as a linear resonator.

The exciting means 20 is formed by a semiconductor laser 21 foroutputting a laser beam which serves as the excitation light 10, and acondensing lens 22 for condensing the excitation light 10 onto thesolid-state laser medium 23. The semiconductor laser 21 is enclosed in acan package. The exciting means 20 directs the excitation light 10 tothe resonator 30 along a direction parallel to the optical axis A of theresonator 30 from the back side 24 b of the concave mirror 24.

The resonator 30 is structured such that only one beam waist of thelasing light 7 is formed within the resonator 30 (on the mode-lockingelement in this embodiment).

The resonator length L and a curvature of a concave surface 24 a of theconcave mirror 24 are set in a range where the beam waist of the lasinglight 7 is formed on the mirror surface 25 a of the saturable absorbermirror 25, and the beam diameter D₁ of the lasing light on the mirrorsurface 15 a is not more than 100 μm. In particular, the resonatorlength L is in a range from 1 mm to 14 mm. The solid-state laser medium13 is disposed such that the average beam diameter D₂ of the lasinglight within the solid-state laser medium 13 is not more than 200 μm.

Further, a Brewster element 26 for making the polarization direction P₀of the lasing light 7 parallel to the c-axis of the solid-state lasermedium 23 is disposed between the solid-state laser medium 23 and thesaturable absorber mirror 25 within the resonator.

An example of more specific structure of the mode-locked laser device 3will now be described. As the solid-state laser medium 23, a Nd:YVO₄medium is used, which is produced by doping Nd ion to a substrate ofYVO₄.

The Nd:YVO₄ medium is doped with 3 at % of Nd ion, has a thickness of 1mm, and is cut such that the a-axis thereof is parallel to the opticalaxis. The both sides of the Nd:YVO₄ medium are provided with a coatingthat well transmits either of the excitation light 10 with a wavelengthof 809 nm and the lasing light 7 with a wavelength band of 1064 nm.

As the saturable absorber mirror 25, one having a transmittance of 0.3%,a modulation depth of 1.2%, and a saturation fluence of 90 μJ/cm² isused.

The mirror surface (concave surface) 24 a of the concave mirror 24 has acurvature radius of 10 mm and a thickness t of 1 mm. Both of the mirrorsurface 24 a and the planer surface (back side) 24 b are provided with acoating that well transmits the excitation light 10, and the mirrorsurface 24 a is further provided with a coating that well reflects thelasing light 7.

The concave mirror 24 and the SESAM 25 are disposed such that theresonator length L defined between the mirror surface 24 a of theconcave mirror 24 and the mirror surface 25 a of the SESAM 25 is 9.1 mm(in the air). The Nd:YVO₄ medium 23 is disposed at a distance d₂ of 0.3mm from the concave mirror 25.

The Brewster element 26 is made of quartz having a thickness of 0.4 mm,and is disposed between the Nd:YVO₄ medium 23 and the SESAM 25 at anangle of 55.4° with respect to the resonator axis A.

As the semiconductor laser 21, a broad-area semiconductor laser having awavelength of 809 nm, an emission light width of 100 μm and a power of 2W is used. As the condensing lens 22, a lens that condenses theexcitation light 10 onto the Nd:YVO₄ medium 23 to provide a beamdiameter of 130 μm with the light being polarized in a directionparallel to the c-axis of the Nd:YVO₄ medium 23 is used, and a specificexample thereof is SELFOC® lens manufactured by Nippon Sheet Glass Co.,Ltd.

In the above-described structure, the lasing beam diameter D₁ on theSESAM 25 is 70 μm, and the average lasing beam diameter D₂ within theNd:YVO₄ medium 23 is 168 μm.

In this mode-locked laser device 3, the light with the wavelength bandof 1064 nm excited by the laser beam 10 and emitted by the Nd:YVO₄medium 23 resonates between the concave mirror 24 and the SESAM 25 andis mode-locked by the SESAM 25, and then is outputted by thetransmission-type SESAM 25 as the pulse laser 8. The mode-locked laserdevice having the above-described structure provides a recurrencefrequency of 16 GHz, a pulse width of 7 psec. and an average power of140 mW.

The respective devices of the above-described embodiments have a simplestructure including the mirrors only at the opposite ends thereof, inwhich only one beam waist of the lasing light is formed within theresonator. Further, the exciting means of each device can be formed onlyby a semiconductor laser and a single lens. Therefore, the devices canbe made compact and stable.

Although Yb:Y₂O₃ or Nd:YVO₄ is used as the solid-state laser medium inthe specific examples of the above embodiments, a solid-state lasermedium in combination with one of various substrates doped with Nd or Ybion, such as Nd:GdVO₄, Nd:YAG, Nd:glass, Yb:YAG, Yb:KY(WO₄)₂,Yb:KGd(WO₄)₂, Yb:Gd₂SiO₅, Yb:Y₂SiO₅, can be used. Further, the ion to bedoped is not limited to Nd or Yb ion, and any of rare earth ions isapplicable.

The mode-locked laser device of the present invention has a Fabry-Perotresonator structure. Therefore, the number of parts forming themode-locked laser device of the invention is smaller than conventionaldevices that require a large number of parts. This allows accomplishinga compact, less expensive and highly stable mode-locked laser device.Further, by limiting the beam diameter of the lasing light on themode-locking element to not more than 150 μm and the beam diameter ofthe lasing light within the solid-state laser medium to not more than200 μm, practically sufficient lasing efficiency can be achieved.

Moreover, with the structure having the mode-locking element and thesolid-state laser medium disposed in the vicinity of the beam waist, thebeam diameter of the lasing light of not more than 150 μm on themode-locking element and the beam diameter of the lasing light of notmore than 200 μm within the solid-state laser medium can easily beaccomplished.

The other mode-locked laser device of the present invention has a simplestructure including a resonator structured as a linear resonator,wherein the resonator includes mirrors only at the opposite endsthereof, and only one beam waist of the lasing light is formed withinthe resonator. Therefore, the number of parts forming the mode-lockedlaser device of the invention is smaller than conventional devices thatrequire a large number of parts. This allows accomplishing a compact,less expensive and highly stable mode-locked laser device. Further, bydisposing the solid-state laser medium and the mode-locking element inthe vicinity of the beam waist such that the distance between thesolid-state laser medium and the mode-locking element is less than ½ ofthe resonator length, the beam diameters of the lasing light within thesolid-state laser medium and on the mode-locking element can be madesmall enough to achieve practically sufficient lasing efficiency.

By forming the mode-locking element with a saturable absorber mirrorthat forms one of the ends of the resonator, the number of parts formingthe device can further be reduced, thereby making the device morecompact.

By providing the resonator mirror, which forms one of the ends of theresonator, with a coating that has a negative group velocitydistribution to provide a zero or less group velocity distribution forthe entire resonator, soliton mode locking can be induced within theresonator, thereby achieving pulsed light with a shorter pulse width.

By forming the exciting means so as to apply the excitation light to theresonator along a direction substantially parallel to the optical axisof the resonator from the back side of the concave mirror, the overlapbetween the excitation light and the lasing light can be increased,thereby improving lasing efficiency.

By making the exciting means having a simple structure that includesonly a semiconductor laser and a single lens, the size of themode-locked laser device can further be reduced.

1. A mode-locked laser device comprising: a Fabry-Perot resonator; amode-locking element disposed within the resonator; a solid-state lasermedium disposed within the resonator; and an exciting means for applyingexcitation light to the solid-state laser medium, wherein opposite endsof the resonator, the mode-locking element and the solid-state lasermedium are disposed to provide an average beam diameter of lasing lightof not more than 150 μm on the mode-locking element and an average beamdiameter of the lasing light of not more than 200 μm within thesolid-state laser medium, wherein the exciting means applies theexcitation light to the resonator from a direction crossing the opticalaxis of the resonator, wherein the exciting means directs the excitationlight directly to the solid-state laser medium; and wherein the excitingmeans is equipped with a focusing lens, the focusing lens focuses theexcitation light and causes the excitation light beam to enter the facetof the solid laser medium at an angle.
 2. The laser device of claim 1,wherein the solid-state laser medium is a laser crystal.
 3. The laserdevice of claim 1, wherein the excitation light beam enters the facet ofthe solid-state laser medium at an oblique angle.
 4. A mode-locked laserdevice comprising: a Fabry-Perot resonator; a mode-locking elementdisposed within the resonator; a solid-state laser medium disposedwithin the resonator; and an exciting means for applying excitationlight to the solid-state laser medium, wherein opposite ends of theresonator, the mode-locking element and the solid-state laser medium aredisposed to provide an average beam diameter of lasing light of not morethan 150 μm on the mode-locking element and an average beam-diameter ofthe lasing light of not more than 200 μm within the solid-state lasermedium, wherein the solid-state laser medium and the mode-lockingelement are disposed in the vicinity of a beam waist of lasing lightwith a distance between the solid-state laser medium and themode-locking element being less than ½ of the length of the resonator,wherein the exciting means applies the excitation light to the resonatorfrom a direction crossing the optical axis of the resonator, wherein theexciting means directs the excitation light directly to the solid-statelaser medium; and wherein the exciting means is equipped with a focusinglens, the focusing lens focuses the excitation light and causes theexcitation light beam to enter the facet of the solid laser medium at anangle.
 5. The laser device of claim 4, wherein the solid-state lasermedium is a laser crystal.
 6. The laser device of claim 4, wherein theexcitation light beam enters the facet of the solid-state laser mediumat an oblique angle.